The present disclosure proceeds from a circuit arrangement for the operation of semiconductor light sources, of the generic type described in the main claim.
In many cases, state-of-the-art circuit arrangements for the operation of semiconductor light sources are not switched in the conventional manner, wherein they are turned on by the switching-in of the mains voltage and turned off by the switching-out of the mains voltage, but are permanently connected to the mains voltage, and are switched by means of a data bus such as, e.g. a DALI bus. The fact that these circuit arrangements are permanently connected to the mains voltage raises a problem which is known from the prior art. As a result of stray capacitances, the AC mains voltage can generate a small current in the semiconductor light sources, which causes the semiconductor light sources to glow, at least in part. Particularly in a dark environment, this glowing can be clearly perceived, and is undesirable. The current responsible for the glowing of semiconductor light sources is described hereinafter as the glow current IG. From the prior art, measures are known which are intended to attenuate the glowing of semiconductor light sources in a switched-out circuit arrangement.
FIG. 2 shows a voltage UEWN which, notwithstanding the switching-out of a circuit arrangement 100 for the operation of semiconductor light sources, is present on the LED string 55, and results in the glowing of the LEDs 5 in the LED string 55. This voltage flows via stray capacitances in the LED string 55, although the circuit arrangement 100 for the operation of semiconductor light sources is not actively in service. This voltage can induce a small current in the light-emitting diodes 5 (typically of a value of 500 μA-1,000 μA), which causes the latter to glow. A glowing of the light-emitting diodes 5, at least in darkness, is visible with effect from a light-emitting diode current of 1 μA.
From FIG. 3, a known method is inferred for the reduction of the glowing of semiconductor light sources.
FIG. 3 shows a circuit arrangement according to the prior art, which already reduces the glowing of the LEDs 5. FIG. 3 represents the output section of the circuit arrangement in the switched-out state, with the semiconductor light sources glowing. The two output conductors LED+ and LED− herein are short-circuited on the input side, on the grounds that, for the e.m.f. UEWN, the interconnection of the circuit arrangement at this point acts in the manner of a short-circuit.
From the prior art it is known that, between a DC voltage converter and the output terminal of the circuit arrangement, a diode 1 is connected in series. In itself, this substantially reduces the glow current, as practically no more current can flow in the blocking direction of the diode. The diode must be appropriate for this function, and must show the smallest possible stray capacitance.
In the light-emitting diode string 55, a protective diode 7 is connected in an antiparallel arrangement with each light-emitting diode 5, which is intended to protect the light-emitting diode 5 from excessively high blocking voltages. Light-emitting diodes are known to be highly sensitive to high blocking voltages, and can be easily destroyed as a result. Consequently, in practically every commercial light-emitting diode package, a protective diode 7 is connected to the LED chip 5 in an antiparallel arrangement. State-of-the-art light-emitting diodes are high-power modules which, on the grounds of their high power conversion capacity, generate substantial quantities of waste heat. As a result, these modules are customarily fitted to “metal-core printed boards”. These are printed circuit boards which are essentially comprised of a good thermally-conductive sheet metal, generally aluminum or copper. A very thin insulating layer is applied to this sheet metal to which, in turn, known printed conductors are applied. As a result of the limited thickness of the insulating layer, very good thermal conduction to the metal core, i.e. to the sheet metal, is provided. Waste heat generated on the light-emitting diodes 5 can thus be evacuated very effectively. However, this thermal advantage is also associated with an electrical disadvantage: as a result of the limited thickness of the insulating layer, the entire arrangement acts as a capacitor, and specifically as a Y-capacitor, as the sheet metal, in the majority of arrangements, is grounded. These stray capacitances are represented in the circuit diagram in FIG. 3 as capacitors 9. Via these capacitors 9, a glow current can flow to ground, even with the circuit arrangement in the switched-out state.
In order to further reduce the glow current flowing in the light-emitting diode string 55, a MOSFET S1 is arranged between the DC voltage converter and the output terminal 124 which, during the operation of the circuit arrangement for operating semiconductor light sources, is switched-in, and is likewise switched-out, when the circuit arrangement for operating semiconductor switches is switched-out. This MOSFET S1 thus further suppresses the glow current in the forward direction of the light-emitting diodes 5. The diode 3 represented in FIG. 3 is the body diode of the MOSFET S1. A varistor 13 is connected in parallel with the drain-source gate of the MOSFET S1, in order to protect the MOSFET S1 against overvoltage pulses. Between the MOSFET S1 and the output terminal 124, a Y-capacitor 11 is arranged in the ground connection, which likewise reduces the glowing of the light-emitting diodes 5.
However, even in this known circuit arrangement, a glow current IG, albeit weak, continues to flow in the light-emitting diodes 5. This is essentially attributable to the drain-source capacitance of the MOSFET switch S1 and, notwithstanding careful selection, also to the rather low resistance value and the high capacitance value of the varistor 13 which, even upon the application of a low voltage thereto, shows a rather low resistance value and a rather high stray capacitance. For technological reasons, the characteristic performance of available varistors is only conditionally suitable for the present application.